Asphaltenes constitute high molecular weight constituents of crude oils that are insoluble in n-heptane and soluble in toluene. They contribute to the stabilization of the water-in-oil emulsions formed during crude oil recovery and hinder drop-drop coalescence. As a result, asphaltenes unfavorably impact water-oil separation processes and consequently oil production rates. In view of this there is a need to better understand the physicochemical effects of asphaltenes at water-oil interfaces. This study elucidates aspects of these effects based on new data on the interfacial tension in such systems from pendant drop experiments, supported by results from nuclear magnetic resonance (NMR) and dynamic light scattering (DLS) studies. The pendant drop experiments using different asphaltene concentrations (mass fractions) and solvent viscosities indicate that the interfacial tension reduction kinetics at short times are controlled by bulk diffusion of the fraction of asphaltenes present as monomer. At low mass fractions much of the asphaltenes appear to be present as monomers, but at mass fractions greater than about 80 ppm they appear to aggregate into larger structures, a finding consistent with the NMR and DLS results. At longer times interfacial tension reduction kinetics are slower and no longer diffusion controlled. To investigate the controlling mechanisms at this later stage the pendant drop experiment was made to function in a fashion similar to a Langmuir trough with interfacial tension being measured during expansion of a droplet aged in various conditions. The interfacial tension was observed to depend on surface coverage and not on time. All observations indicate the later stage transition is to an adsorption barrier-controlled regime rather than to a conformational relaxation regime.
In an earlier study, oil-water interfacial tension was measured by the pendant drop technique for a range of oil-phase asphaltene concentrations and viscosities. The interfacial tension was found to be related to the relative surface coverage during droplet expansion. The relationship was independent of aging time and bulk asphaltenes concentration, suggesting that cross-linking did not occur at the interface and that only asphaltene monomers were adsorbed. The present study extends this work to measurements of interfacial rheology with the same fluids. Dilatation moduli have been measured using the pulsating droplet technique at different frequencies, different concentrations (below and above CNAC), and different aging times. Care was taken to apply the technique in conditions where viscous and inertial effects are small. The elastic modulus increases with frequency and then plateaus to an asymptotic value. The asymptotic or instantaneous elasticity has been plotted against the interfacial tension, indicating the existence of a unique relationship, between them, independent of adsorption conditions. The relationship between interfacial tension and surface coverage is analyzed with a Langmuir equation of state. The equation of state also enabled the prediction of the observed relationship between the instantaneous elasticity and interfacial tension. The fit by a simple Langmuir equation of state (EOS) suggests minimal effects of aging and of nanoaggregates or gel formation at the interface. Only one parameter is involved in the fit, which is the surface excess coverage Γ∞ = 3.2 molecules/nm(2) (31.25 Å(2)/molecule). This value appears to agree with flat-on adsorption of monomeric asphaltene structures consisting of aromatic cores composed of an average of six fused rings and supports the hypothesis that nanoaggregates do not adsorb on the interface. The observed interfacial effects of the adsorbed asphaltenes, correlated by the Langmuir EOS, are consistent with the asphaltene aggregation behavior in the bulk fluid expected from the Yen-Mullins model.
Cascaded lattice-Boltzmann method (Cascaded-LBM) employs a new class of collision operators aiming to stabilize computations and remove certain modeling artifacts for simulation of fluid flow on lattice grids with sizes arbitrarily larger than the smallest physical dissipation length scale (Geier et al., Phys. Rev. E 63, 066705 (2006)). It achieves this and distinguishes from other collision operators, such as in the standard single or multiple relaxation time approaches, by performing relaxation process due to collisions in terms of moments shifted by the local hydrodynamic fluid velocity, i.e. central moments, in an ascending order-by-order at different relaxation rates. In this paper, we propose and derive source terms in the Cascaded-LBM to represent the effect of external or internal forces on the dynamics of fluid motion. This is essentially achieved by matching the continuous form of the central moments of the source or forcing terms with its discrete version.Different forms of continuous central moments of sources, including one that is obtained from a local Maxwellian, are considered in this regard. As a result, the forcing terms obtained in this new formulation are Galilean invariant by construction. To alleviate lattice artifacts due to forcing terms in the emergent macroscopic fluid equations, they are proposed as temporally semi-implicit and second-order, and the implicitness is subsequently effectively removed by means of a transformation to facilitate computation. It is shown that the impressed force field influences the cascaded collision process in the evolution of the transformed distribution function. The method of central moments along with the associated orthogonal properties of the moment basis completely determines the analytical expressions for the source terms as a function of the force and macroscopic velocity fields. In contrast to the existing forcing schemes, it is found that they involve higher order terms in velocity space. It is shown that the proposed approach implies "generalization" of both local equilibrium and source terms in the usual lattice frame of reference, which depend on the ratio of the relaxation times of moments of different orders. An analysis by means of the Chapman-Enskog multiscale expansion shows that the Cascaded-LBM with forcing terms is consistent with the Navier-Stokes equations. Computational experiments with canonical problems involving different types of forces demonstrate its accuracy.
Manganese dioxide cathodes are inexpensive and have high theoretical capacity (based on two electrons) of 617 mAh g−1, making them attractive for low-cost, energy-dense batteries. They are used in non-rechargeable batteries with anodes like zinc. Only ∼10% of the theoretical capacity is currently accessible in rechargeable alkaline systems. Attempts to access the full capacity using additives have been unsuccessful. We report a class of Bi-birnessite (a layered manganese oxide polymorph mixed with bismuth oxide (Bi2O3)) cathodes intercalated with Cu2+ that deliver near-full two-electron capacity reversibly for >6,000 cycles. The key to rechargeability lies in exploiting the redox potentials of Cu to reversibly intercalate into the Bi-birnessite-layered structure during its dissolution and precipitation process for stabilizing and enhancing its charge transfer characteristics. This process holds promise for other applications like catalysis and intercalation of metal ions into layered structures. A large prismatic rechargeable Zn-birnessite cell delivering ∼140 Wh l−1 is shown.
Previous studies indicated that asphaltenes adsorbed as monomers on oil-water interfaces and the early stage kinetics of the process was controlled by diffusion and hence dependent on oil viscosity. By measuring interfacial tension (IFT) as a function of surface coverage during droplet expansions in pendant drop experiments, it was also concluded that the IFT data could be interpreted with a Langmuir equation of state (EoS), which was independent of oil viscosity, time of adsorption, and bulk asphaltenes concentration. The surface excess coverage was calculated to be ∼0.3 nm(2)/molecule, which suggested adsorption in face-on configuration of asphaltenes monomers at the interface and average PAH core per molecule of about 6 for the asphaltenes investigated, consistent with the Yen-Mullins model. The current study focuses on the kinetics of asphaltenes adsorption at longer times and higher interfacial coverage. Long-term IFT data have been measured by the pendant drop method for different asphaltenes concentrations and for different bulk viscosities of the oil phase (0.5-28 cP). The data indicate that when coverage reaches 35-40%, the adsorption rates slow down considerably compared to the diffusion-controlled rates at the very early stages. The surface pressure increase rate (or IFT decrease rate) at these higher coverages is now independent of oil viscosity but dependent upon both surface pressure itself and asphaltene monomer concentration. The long-term asymptotic behavior of surface coverage is found to be consistent with the predictions from surface diffusion-mediated random sequential adsorption (RSA) theory which indicates a linear dependency of surface coverage on 1/√t and an asymptotic limit very close to 2D random close packing of polydispersed disks (85%). From these observations RSA theory parameters were extracted that enabled description of adsorption kinetics for the range of conditions above surface coverage of 35%.
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